1. Field of the Invention
The present invention relates generally to electronic synthesis of sounds, and more particularly, to the digital synthesis of background sounds employed in training simulators which simulate antisubmarine-warfare episodes in real time.
2. Description of the Related Art
Simulators are widely employed today to train personnel in the operation of complex mechanical, electrical and electromechanical systems. A simulator acquaints personnel being trained with problems they will confront in real-life situations without exposing them to the actual risks which they would face in such situations. Such risks would be enhanced greatly by the fact that the personnel confronting them often have little, if any,experience. The use of training simulators is also cost-effective. If untrained or minimally trained personnel were, by necessity, forced to gain experience on actual operating systems, the likelihood of damage to the system or of personnel injury would be much greater precisely because serious consequences could ensue from mistakes or errors in judgment. The use of a simulator, on the other hand, permits inexperienced personnel to learn while they are seemingly performing the tasks for which they are being trained without risk of actual potentially disastrous consequences.
Training on a simulator is most effective if the environment which is created and the problems which are posed are as close to real life as are possible. Among the elements which add to the realism of the simulation are resemblance of the simulator equipment to actual equipment, close approximation of sensory information (including sights and sounds) provided to operating personnel in real-life situations, and the posing of problems which are very similar to those with which the trainees will be confronted after their training programs have been successfully completed.
The above is no less true in the training of antisubmarine-warfare personnel than is the case in any other training situation. Antisubmarine warfare today is characterized by high levels of tactical sophistication and the use of complex equipment.
In a typical antisubmarine-warfare episode, a patrol craft (typically an airplane) drops sonobuoys into the water. These sonobuoys carry hydrophones which pick up acoustic signals propagating through the surrounding water. Each sonobuoy also includes a radio transmitter which relays the acoustic signals picked up back to the patrol craft. Each sonobuoy can be set to transmit on a particular radio-frequency channel. Typically, the receiver on the patrol craft receives 36 such channels, allowing the signals from 36 out of a possible 99 sonobuoys to be monitored.
It is apparent that any simulator which is to provide a realistic simulation of an antisubmarine-warfare episode has a formidable task. The simulator must generate a number of different signals, each with different amplitude and spectral characteristics, many of which are episode related. The simulator must provide a synthesized version of a signal from a sonobuoy which depends on the location of the sonobuoy and of sound sources, relative movement between the two (which causes Doppler effects), and the operating condition of the sonobuoy.
These difficulties have resulted in the use of sophisticated computers to produce such complex simulations. Such a simulation computer is programmed with a vast amount of information and instructions corresponding to a number of antisubmarine-warfare episodes. The simulation computer keeps track of a simulation episode as defined by a training instructor.
The sounds detected by any particular sonobuoy depend on a number of factors, including the location and type of sonobuoy, the presence of noise sources in the water around the sonobuoy, the orientation of these sound sources with respect to the sonobuoy, and the operating condition of the sonobuoy.
A sonobuoy picks up ambient sea sounds, such as those produced by aquatic creatures. These ambient sounds can include, for example, those generated by humpback whales, bottlenose porpoises, and snapping shrimp. Also, sonobuoys and their hydrophone components can generate acoustic noises. For example, one of these sounds is generated when a hydrophone is lowered from a sonobuoy housing to a desired depth. Another sound is that produced by a hydrophone having a faulty lowering mechanism. Such a faulty lowering mechanism results in the hydrophone being xe2x80x9chungxe2x80x9d, causing it to bang against the side of the sonobuoy housing. Additional sources of sounds present in antisubmarine warfare are produced by explosions, hulls crumpling, and submarine control devices (such as rudders and ballast tanks).
It is thus clear that ambient sea sounds, including sounds produced by man-made devices, must be produced in antisubmarine-warfare simulators if authentic antisubmarine-warfare episodes are to be simulated.
In previous antisubmarine-warfare simulators, such sounds are generated by analog means such as analog oscillator circuits and analog recorders. The analog approach, however, exhibits many disadvantages. Specifically, analog means and the sound signals they produce are subject to drift and the adverse effects of component aging. Analog tape recorders have maintenance problems and allow very little control over the sounds which they reproduce. Real sounds that are taped, by their very nature, typically contain other sounds which interfere with those which are to be simulated. For example, a tape of transient sounds generated by a submarine""s maneuvering system typically also contains submarine-generated sounds such as those from the submarine""s power plant as well as background sea sounds. These other signals appear as spurious signal components which interfere with the sounds required for simulation of a particular tactical situation.
The digital synthesis of signals in accordance with the present invention results in much greater control over the synthesized background sound signals and allows the reproduction of sounds that are not contaminated by other, spurious components. The digital signal synthesizer of the present invention can be programmed to introduce such effects as Doppler shift, multipath, and directional characteristics. In most cases, such signal modifications can be accomplished in the present invention without any change in hardware. Rather, they can be effected by changes in the software. Because digital circuits are not subject to drift and other adverse effects which are present in analog circuits, and because digital circuits have no moving parts, initial calibration and subsequent maintenance are greatly simplified in the present invention.
The present invention is an underwater background acoustic signal synthesizer and method, which generates, on a multiplexed basis, digital signals corresponding to background or ambient antisubmarine-warfare sounds. These digitally synthesized signals are multiplexed onto a bus which conveys them to a multiplier which digitally imparts to them directional characteristics. The digital signals are then applied to a demultiplexer and digital-to-analog converter, which demultiplexes them, converts them to analog form, and assigns them to appropriate channels of an audio system. These channels correspond to the selectable channels of receiving equipment on board the patrol craft. The audio system thus functions as a dummy radio-frequency receiver, and provides audible signals corresponding to those which would be presented to a crew monitoring such a receiver during an actual antisubmarine-warfare episode.
All of the signals which are synthesized by the apparatus and method of the present invention are generated in the digital domain. To reduce the amount of hardware necessary to generate the large number of digital signals required for a realistic simulation, multiplexing is extensively employed.
In general terms, the system of the present invention comprises a first computer, a second computer, a hardware controller, a synthesizer and an output stage. The first computer, which typically is the main computer of a larger simulation system, generates first digital signals having first data signals, first address signals and first control signals. The second computer has a stored program. The second computer, which is responsive to the first computer, generates second digital signals including second data signals, second address signals and second control signals in accordance with the first digital signals and the stored program. The hardware controller, which is responsive to the second computer, generates third digital signals including third data signals, third address signals and third control signals in accordance with the second digital signals.
The synthesizer, which is responsive to the hardware controller, synthesizes fourth digital signals in accordance with the third digital signals. The fourth digital signals comprise samples of digitally synthesized hydrophone-lowering sounds. Finally, the output stage, which is responsive to the synthesizer, converts the fourth digital signals to analog output signals.
The first computer can be a mainframe computer. The second computer can be a microprocessor, associated with a first bus coupled to it, a first memory coupled to the first bus, a first interface for allowing communication between the first computer and the first bus, and a second interface for allowing communication between the first bus and the hardware controller. The first interface allows direct memory access communication between the first and second computers. The second interface allows direct memory access communication between the second computer and the hardware controller.
The first memory comprises a random-access memory. The first memory stores the stored program. The first memory also stores certain of the first digital signals.
The third data signals comprise first amplitude data words and first frequency data words. The third control signals comprise first timing signals. The synthesizer comprises a line generator, which is responsive to the third digital signals, for generating first digital sinusoids. The first digital sinusoids comprise output digital data words having sinusoidally time-varying magnitudes.
The line generator comprises a sample memory, a first addressing stage, a first timing stage, and a second output stage. A sample memory stores sinusoid samples. The first addressing stage, which is responsive to the third digital signals, reads the sinusoid samples from the sample memory. The first timing stage, which is responsive to the third digital signals, controls the reading of the sinusoid samples by the first addressing stage. The second output stage, which is responsive to the third digital signals, provides the sinusoid samples read from the sample memory. The first addressing stage includes a first port, a first storing stage, a first converting stage, and a first reading stage. The first port receives first frequency data words contained in the third data signals. The first storing stage stores the first frequency data words. The first converting stage converts the first frequency data words into first frequency address words. Finally, the first reading stage reads the sinusoid samples from the sample memory in accordance with the first frequency address words.
The first converting stage can comprise a second storing stage for storing the first frequency address words, and a first incrementing stage for incrementing the stored first frequency address words in accordance with the first frequency data words. Moreover, the second output stage can comprise a port, a storing stage, a reading stage and a multiplying stage. The port receives the first amplitude data words in the third data signals. The stage stores these first amplitude data words, while the reading stage reads the first amplitude data words from the storing stage. Finally, the multiplying stage provides first product words by multiplying the sinusoid samples and the first amplitude data words.
The synthesizer stage can further include a stage for generating first digital noise words in accordance with the third digital signals. A first port receives the first digital noise words, and a first multiplying stage provides first product words by multiplying the first digital noise words and the sinusoid samples.
The synthesizer can include a noise-generator stage having a first timing stage and a pseudorandom binary number generator stage. The first timing stage provides first timing signals in accordance with the third digital signals, and the pseudorandom binary number generator stage generates pseudorandom binary numbers in accordance with the first timing signals. The noise generator can further include a first port stage for receiving first impulse-response data words and a digital-filter stage, which is responsive to the pseudorandom binary number generator stage, for producing first filtered noise words by convolving the pseudorandom binary numbers and the first impulse-response data words. The noise generator stage can also include a second port stage for receiving first amplitude data words, a second multiplier stage and a second output stage. The first multiplier stage provides first digital noise words by multiplying the first filtered noise words and the first amplitude data words, and the second output stage presents these first digital noise words to the first output stage. The noise generator stage can also include a second output stage for converting the pseudorandom binary numbers to a first random number output.
The synthesizer stage can also include a directionality-imparting stage for imparting directional characteristics to the fourth digital signals. Specifically, the directionality-imparting stage can include a directionality-coefficient generator stage and a first multiplying stage. The directionality-coefficient generator provides directionality-coefficient data words in accordance with the third digital signals. The first multiplying stage imparts directionality characteristics to the fourth digital signals in accordance with the directionality-coefficient data words.
With respect to the multiplex aspect of the present invention, the first output stage can comprise a plurality of first output channels, and a routing stage for routing the fourth digital signals to these plurality of first output channels in accordance with the third digital signals. In addition, the first output stage can include a digital-to-analog converter stage converting the fourth digital signals to analog output signals, and a first analog output channel stage for receiving the analog output signals. In addition, the first output stage can further include a low-pass filter stage,interposed between the digital-to-analog converter stage and the first analog output channel stage, for low-pass filtering the analog output signals.
The present invention generates hydrophone-lowering sounds.
The hydrophone-lowering sound comprises a high-pass-filtered noise and a low-pass filtered noise, which are summed together to produce a summed noise. The summed noise has an envelope of random pulsation. The high-pass-filtered noise contains spectral components distributed upward from approximately 200 to 600 Hertz, whereas the low-pass-filtered noise contains spectral components distributed below approximately 800 Hertz.
The synthesizer stage comprises a hydrophone-lowering stage for synthesizing the hydrophone-lowering sound. The hydrophone-lowering stage can include a first stage for generating high-pass-filtered digital noise words in accordance with the third digital signals, a second stage for generating low-pass-filtered digital noise words in accordance with the third digital signals, and a third stage for providing summed noise words by summing the high-pass-filtered digital noise words and the low-pass-filtered digital noise words.